Preallocate additional plus addresses if below minimum plus email allocation

For a user with preallocation turned on, we expect each of their clients to have a minimum number of reserved addresses to choose from. The minimum is currently set to 10 addresses. However, we've observed that although these users start with enough addresses, more are not requested as they are used up.

This CL adds a call to maybe allocate more addresses in `AllocatePlusAddressSynchronously` which ensures that the pool of available plus addresses is replenished if it falls below the minimum threshold, whether due to user consumption or address expiration

Note: The call to fetch additional addresses is an asynchronous one, so  `AllocatePlusAddressSynchronously` may be a misnomer. However, because the functions returns the address synchronously, we will not change the name of the function.

Bug: crbug.com/384481217
Change-Id: I153f16fc03a827ec7c6e35f5e5a9042cf57eea8b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6617481
Reviewed-by: Jan Keitel <jkeitel@google.com>
Commit-Queue: Fikre Mengistu <fikremengistu@chromium.org>
Reviewed-by: Matt Reichhoff <mreichhoff@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1472391}